1. Field of the Invention
The present invention relates to a lithographic apparatus, a projection apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The known lithographic apparatus comprises a control system to control the position of the substrate table. This control system is not only configured to place the substrate table in a desired position on the basis of a reference signal, but also to follow the lens in a horizontal direction in order to minimize image errors caused by movements of the lens, which movement are for instance caused by vibrations in the lithographic apparatus. For this reason a projection system position signal is added to the substrate table position reference signal, so that the controller error is adapted for movements of the lens. Usually the projection system position signal is obtained by a interferometer measuring system. To further increase the accuracy in following the movements of the lens, the control system comprises a feed-forward branch which adds a feed-forward signal representative for the acceleration of the lens to a output signal of the controller unit of the control system. The feed-forward signal is obtained by accelerometers arranged on the lens. The feed-forward signal is conditioned by a feed-forward filter unit. This feed-forward filter unit may comprise analogue filtering of the feed-forward signal and may also comprise digital filters to shape the feed-forward correctly.
A disadvantage of the known control system is that the filtering of the feed-forward signal introduces phase lag/delay in the feed-forward branch, which results in that the actual position of the substrate table lags behind the actual position of the lens, and hence creating a servo error/lens tracking error. This servo error/lens tracking error may be countered by adding lead-lag filters in the feed-forward branch, which introduce a phase advantage at the dominant lens resonance frequencies. However, this is only possible for a limited number of frequencies and as a consequence the response becomes worse at other frequencies. For this reason, the performance that can be gained with this strategy reaches its limits when the number of lens resonance frequencies increases and/or the position accuracy requirements become higher. Furthermore, the lead-lag filters are highly lens dependent, and therefore often have to be tuned by hand, which increases the risk on a poor response on the movements of the lens.